1. Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Avenue, Ann Arbor, MI 48109, USA.
2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
3. Harvard-Smithsonian Center for Astrophysics, Harvard University, Cambridge, MA 02138, USA.
4. Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK.